Injection molding devices are used to make a wide variety of molded plastic items, such as containers or toys. In one type of device, for example, solid thermoplastic pellets are stored in a hopper. From the hopper, the pellets are conveyed through a passage by a screw which pushes the pellets in an augering motion. The motion of the screw and heaters positioned around the passage melting the pellets into a molten plastic resin that is injected through a manifold into one or more nozzle assemblies. Nozzle assemblies are available in different shapes, sizes, and configurations, but typically have an elongated body with a flow passage having a receiving end facing the manifold for receiving the molten plastic resin, and an outlet end with a gate opening which has a centered, tapered tip therein. This tip is sometimes referred to as a "needle." A heating element is provided near the passage in the nozzle assembly to help maintain the resin flow.
The plastic resin enters the flow passage from the receiving end, and flows to the gate opening at the tip, and then into a cavity. The cavity in the mold is filled when the screw is turned to a specified amount and then the resin is injected into the cavity. The molded plastic object is allowed to cool. The mold is opened and ejector pins in the mold are used to eject the molded plastic object.
The nozzle assembly is positioned within an opening in a mold which includes a nozzle plate and/or a cavity plate. Each of these plates may be cooled by a water line, so there is a thermal difference at points of contact between the nozzle assembly and the opening. It is desirable to maintain the heat at a specific temperature on the resin within the nozzle assembly, and not to lose too much heat to the opening through the contact points. The heating element, in some embodiments, is positioned around a portion of the nozzle body and is separated from the nozzle plate and the cavity plate by air which serves as an acceptable thermal insulator.
The nozzle body may directly contact the opening in the mold, for example, at the nozzle plate at one location, and at the cavity plate at a seal-off area. The nozzle assembly may contact the nozzle plate to provide support within the opening, and may contact the cavity plate at the seal-off to prevent resin from leaking into the heater area. It is desirable to keep the heat on the resin within the nozzle assembly, and not to lose too much heat to the housing where the nozzle assembly and the housing are in contact.
An attempt has been made to insulate the nozzle assembly from the opening in the cavity plate by pressing a titanium ring over the nozzle assembly at the seal-off area. While titanium has fairly good heat insulating properties (about 12 btu/ft hr .degree.F.), it is rather soft when subjected to the operating temperatures and pressures. When the titanium softens, it can deform so that resin can escape from the seal-off area.